1. Field of the Invention
The present invention relates generally to electrical function generators, and relates specifically to function generators of the non-linear type. Such a function generator characteristically produces an electrical output signal which follows a varying input signal, but which is related to the input signal according to a desired function which varies in a predetermined manner as the input signal varies. More specifically, the invention relates to that type of non-linear function generator which has particular utility in non-linear process control systems or controllers, wherein the function generator provides the desired non-linear relationship between a deviation or error signal and the control action produced in accordance with that error signal.
2. Description of the Prior Art
Non-linear function generators as defined above are known in the art. Such a generator characteristically includes an input connection, an output connection, and biased amplifying and/or attenuating means connected between those connections. The input connection is arranged to receive a varying input signal. As long as this signal lies within a predetermined signal range or band which is established by the noted biasing, the apparatus produces an output signal which follows the input signal with a desired gain. In other words, the output signal is then proportional to the input signal according to a desired proportionality constant. When the input signal lies outside of the noted band, the output signal is made to follow the input signal with a different, usually higher, gain.
When such a function generator is employed in one of the noted non-linear control systems, the generator input signal is usually the controlled process variable deviation or error signal. Such a signal is generally obtained by comparing a set point signal, representing a desired value of the controlled variable, with a signal representing the instantaneous measured or actual value of the variable. The noted band, within which the output signal follows the input signal with the first-noted gain, is then the so-called deadband of the control system. This output signal is applied to a control device or controller portion of the system to cause that device to appropriately affect or influence the value of the variable as necessary to minimize the error signal.
In the function generator application being described, the gain for the deadband is made desirably low. Thus, when the actual value of the variable is sufficiently close to the desired value that the error signal lies within the deadband, the generator output signal follows the error signal with this low gain, and so causes the corrections which the control device makes to the value of the variable to be suitably small.
When the actual value of the variable becomes so different from the desired value that the error signal lies outside of the deadband, the higher gain with which the output signal then follows the error signal causes the corrections made by the control device to be suitably larger, to the end of hastening the return of the actual value of the variable to the desired value therefor. Thus, the controlled non-linearity of the generator output with respect to the error signal provides the increased corrective action needed when the actual value of the controlled variable departs from the desired value by what is considered to be an excessive extent.
An example of the known non-linear function generators and their use in non-linear control systems as just described is found in the Shinskey U.S. Pat. No. 3,794,817. The non-linear function generator of the control system of that patent is shown in FIG. 3 thereof, and is seen to include a pair of transistors and an operational amplifier.
Another typical example of the known non-linear function generators for use in non-linear controllers is the generator disclosed in the Porawski U.S. Pat. No. 3,851,259. The generator of that patent utilizes three operational amplifiers and two diodes to produce a non-linear output signal. Still another such generator, but employing vacuum tubes and diodes, is shown in FIG. 3 of the Roper et al. U.S. Pat. No. 2,895,502. Other examples of more general purpose function generators with which I am familiar are those disclosed in the Harder U.S. Pat. No. 2,697,201, the Herzog U.S. Pat. No. 3,158,739, and the Tsuda U.S. Pat. No. 3,621,227.
Each of the known non-linear function generator arrangements with which I am familiar, as typified by the arrangements of the noted patents, possesses one or more characteristics or shortcomings which has made it less than desirable for use in certain applications. For example, a function generator of the type which is shown in the Porawski patent has the disadvantage of not providing an adjustable gain for the deadband. A function generator of the type which is shown in the Roper et al patent does not lend itself to present day construction techniques. The function generators which are shown in the Harder, Herzog, and Tsuda patents are not of forms which are readily arranged or practically constructed to provide the type of non-linear output signal discussed above.
Finally, a function generator of the type which is shown in FIG. 3 of the Shinskey patent has the disadvantage that the change between the deadband gain and the higher gain, for error signals outside of the deadband, does not occur abruptly, as is desirable. Instead, as shown in FIG. 2a of the Shinskey patent, the noted change in gain occurs gradually, due to the characteristics of transistors. It has been found that the parameters of the transistors which cause this gradual gain change tend to be unstable, due to temperature dependence and the like, thereby making the transition points between the two gain values unstable. Such instability is undesirable in certain applications, and amounts to a shortcoming of the apparatus providing it.